This invention relates to the introduction of additives into a polymer. More specifically, this invention relates to an apparatus and method for the direct injection of additives into a polymer melt stream.
The addition of additives to molten polymers has been accomplished by several means. One such means is blending the additives and the polymer chips together in the polymer chip dryer or in the storage hopper prior to extruding the polymer chips and the additives into strands for pelletizing. Another method for introducing additives into a polymer melt stream is to inject the additives at the throat, the mixing zones, or the vent of the extruder and to allow the extrusion process to fully blend the additives into the polymer components. A third method of introducing additives into a polymer melt stream involves injecting the additives into static mixing elements located downstream of the extruder to fully blend the additives into the polymer components.
Problems arise, however, in that some additives may be heat sensitive and may also cause polymer degradation or other undesirable reactions with the polymer if blended with the polymer before extrusion into polymer strands. Moreover, some additives such as, for example, zinc stearate, can cause extruder screw slippage.
A way to overcome such problems is to introduce the additives into a polymer melt stream after extrusion of the polymer into strands for pelletizing. One such method is to coat the polymer pellets with the additives after the polymer extrusion process has occurred. A problem arises, however, in that for additives that amount to less than about 1 percent of the concentration of the total polymer product, this method does not generally result in a good uniform dosing of additive to polymer.
A need, therefore, exists for a method of introducing additives into the polymer that overcomes the above-discussed limitations.
It is a primary object of the present invention to introduce one or more additives directly into a polymer melt stream.
Another object of the present invention is to strategically place one or more additives at specific locations within an extruded polymer strand using a thin plate die assembly.
Thus, according to one embodiment of the present invention, there is provided a method of directly injecting one or more additives into a polymer melt stream comprising the steps of supplying a melt flow of a polymeric host material to a die assembly having a thin-plate assembly and injecting at least one additive into at least one predetermined location in a cross-section of the melt flow of the polymeric host material while passing the melt flow of the polymeric host material through the die assembly. The one or more additives is injected into one or more exact locations within the cross-section of the polymeric host to achieve uniform dosing, in the extrusion direction, of the one or more additives within the polymeric host material without homogeneously mixing the one or more additives and the polymeric host material into a single phase.
According to another aspect of the present invention there is provided an apparatus for carrying out the direct injection of one or more additives into the melt flow of a polymeric host material comprising a pumping system, a die assembly having a thin-plate assembly, and a distribution line.
According to yet another embodiment of the present invention there is provided a method of making pellets from polymers comprising the steps of supplying a melt flow of at least one polymeric host material to a die assembly comprising a thin-plate assembly, directing the injection of at least one additive into at least one predetermined location in a cross-section of the melt flow while passing the melt flow through the die assembly to form strands, and cutting the polymer strands into pellets. The resulting pellets have a precise amount of additive dosed at the at least one predetermined location in the cross-section of the polymeric host material. The uniform dosing of the one or more additives is achieved without homogeneous mixing of the additive and the polymeric host material.
By precisely injecting low concentrations of one or more sensitive polymer additives into the polymeric host material at the die, degradation and chemical reactions in the extruder are avoided, handling of additive material is simplified, and uniformity of the additive in the strand of polymeric host material is improved. Moreover, accurate placement of additives in the cross-section of a strand of polymeric host material is achieved.